Flexible circuits are used in the electronics industry as the base materials for fabricating a wide variety of flexible interconnection products such as flexible circuit boards and flex-rigid circuit boards. Flexible circuit boards and flex-rigid circuit boards are used in notebook computers, printers and disc drives, as well as numerous medical devices and consumer products. Flexible circuits are also used for certain advanced applications such as chip-on-flex and fine-line circuit boards. With the electronics industry moving toward thinner, lighter, flexible and more functional products, the demand for flexible circuits continues to increase.
Flexible circuits are conventionally made of a copper layer (copper conductor) sandwiched between two organic polymeric layers. In particular, copper foil is bonded with a substrate, patterned, and a coverlayer is applied over the copper foil. As the name implies, flexible circuits maybe bent and unbent during use. Accordingly, it is desirable for the flexible circuit to possess a high degree of structural integrity in order to maintain electrical properties. Structural integrity provides resistance to mechanical fatigue caused by bending and unbending of the flexible circuit which leads to electrical failure.
The early indications of mechanical fatigue in flexible circuits are characterized by the generation and propagation of microscopic cracks at the surface of the copper foil layer. The microscopic cracks may extend into the thickness or across the width of the copper foil. As flexible circuits are used, the microscopic cracks eventually become cracks of notable size that can traverse the thickness of the copper foil or lead to gauging, wherein a small piece of copper foil at the surface of the copper foil layer is released. This type of damage to the copper foil layer, of course, leads to electrical failure.
The generation and propagation of cracks due to bending is referred to as "fatigue". There are three primary types of fatigue; namely, roll fatigue, flex fatigue, and fold fatigue. Roll fatigue is mainly attributable to two forces on the copper foil of the flexible circuit. Referring to FIG. 1, flexible circuit 10 is moved back and forth as indicated by arrows 12. This action mimics the motion of a disk drive. Arrows 14 represent tensile forces on the flexible circuit 10 (and particularly the copper foil, not shown, therein). Arrows 16 represent compressive forces on the flexible circuit 10 (and particularly the copper foil, not shown, therein). As the flexible circuit 10 is moved back and forth along arrow 12, the tensile forces and the compressive forces move back and forth thereon. The constantly repeated stress imposed by the tensile forces and the compressive forces leads to roll fatigue of the copper foil within flexible circuit 10. Flex fatigue is characterized by holding the flexible circuit at two points and applying force normal to the flexible circuit to a point about half way between the two holding points followed by applying another force normal to the flexible circuit in the opposite (180.degree.) direction. Fold fatigue is characterized by initially holding the flexible circuit with a 135.degree. bend and then folding the flexible circuit to have a 0.degree.-2.degree. bend and then unbending back to 135.degree.. This action mimics the motion of a printer hinge.
The three primary types of fatigue (roll fatigue, flex fatigue, and fold fatigue) are generally caused by high cycle, low strain fatigue. Another type of fatigue is low cycle, high strain fatigue. It is difficult to provide a flexible circuit having resistance to both high cycle, low strain fatigue and low cycle, high strain fatigue.
Referring to FIGS. 2A and 2B, illustrations of microscopic cracks, some of which extend the thickness or the width of the copper foil and some of which do not, are shown. The illustrations are based on photographs taken at a magnification of 1600.times. of copper foil having a thickness of about 18 .mu.m.